Printed electrical resistor



United States Patent PRINTED ELECTRICAL RESISTOR David B. Peck, Williamstown, Mass., assignor to Sprague Electric Company, North Adams, Mass., a corporation of Massachusetts No Drawing. Original application May 16, 1952, Serial No. 288,305, now Patent No. 2,795,680, dated June 11, 1957. Divided and this application April 4, 1955, Serial N 0. 499,239

7 Claims. (Cl. 201-63) This invention relates to the provision of improved electrical devices (particularly resistors) as well as improved inks for the production of such devices. More particularly, the invention relates to the pro-vision of resistors having a resistance in the range of 10-600 ohms and the inks therefor.

Electrical resistors are presently manufactured in many forms. Very recently, however, those designated as printed resistors have obtained a great deal of popularity and have become extensively utilized. These resistors are normally manufactured by laying down on an insulating base, through a silk or steel screen, a suspension of carbon black or graphite in a solution of a resinous binder, followed by removal of the solvent and curing of the resin. Numerous insulating base materials have been used for these devices particularly ceramics, Bakelite and other resinous materials, and glass. In the resistors prepared according to the prior art procedures, instability is often observed because of the greater loads now being imposed and/ or the more critical circuit positions in which the resistors are placed. For example, prior art resistance elements containing carbon as the conducting material in a phenol-formaldehyde resin binder, will not maintain their original resistance value for reasonable operational periods, especially when subjected to broad variations in heat or humidity conditions. Likewise, resistors formed using other resinous binders have been found to lack adequate stability when subjected to varying conditions of temperature, humidity, wear and load.

Another problem normally encountered in electrical elements, such as resistors, stems from the manufacturing procedures employed. Thus, in practice, deposition of the resistor material is normally followed by curing of the resinous binder. One must then screen a protective resin coat over the resistor, cure this coat, solder lead wires to the. usually silvered contact areas, degrease the flux from the soldered joints, apply an outer protective coat of resin, cure this coat, and finally vacuum-wax-impregnate the protective resin coat. These numerous operations may greatly effect the resistance value and other electrical characteristics of the printed resistor. In most cases, the

I resistance value is increased, but at such a non-uniform rate that resistance value tolerances are almost impossible to maintain. Other processing difiiculties such as control of viscosity, suspension uniformity, and variations in the extent of resin polymerization during and following screening, may also be encountered. Thus, resistance values must very often be crudely adjusted by scraping off some of the resistance layer after curing of the resin binder. Applicants are aware that various resins have been used previously for the preparation of resistance inks. These resins include the phenol formaldehyde, urea formaldehyde, melamine formaldehyde, silicone, lineed oil, etc. types. However, the inks produced using these resins are relatively unstable, yield a negative coefiicient of resistance and are in many cases noisy (i. e.,

noises result in electrical circuits employing such resistors in stages where signals are being handled).

It is an object of the present invention to overcome the foregoing and related disadvantages; and to provide printed electrical elements, such as resistors of unusual operational stability, having resistance values of the order of about 10-600 ohms within accurately controlled tolerances. A further object of the invention is to provide inks containing as the conductive material, a combination of carbon and silver flake in an epoxy resin binder.

More specifically, the invention includes printed resistors made by depositing on a ceramic, glass, Bakelite (or other electrically inert) insulating base upon which is deposited an adherent layer containing about 5 to about conducting particles comprising a mixture of silver flake and carbon and from about 95 to 50% resinous binder consisting of a cross-linked epoxy resin (preferably formed from treatment of the epoxy resin with a butylated urea formaldehyde resin). The ratio of carbon to silver flake in the conducting-particle-mixture may vary within wide limits. Prefer-ably, the silver flake should constitute a minimum of about 5% and a maximum of about 85% by weight of the mixture.

In preparing the low resistance inks of the present invention the resin mixture is dissolved in a relatively high boiling solvent and the desired amount of the conductive particle mixture is suspended in this solution. The resin component which effects cross-linking is preferably active for this purpose only at temperatures exceeding about 75 C. or more (e. g. 150 C.).

The carbon particles used in the conductive component of the resistance inks may be any of those normally utilized. Thus, graphite, lamp black, carbon black, etc. may be employed. The silver flake component of the conductive mixture may be obtained by any desirable procedure and can be generally described as pure silver in the form of fine plate-like particles coated with a lubricant as stearic acid. However, in view of its use in an electrical component, it must be of high purity. A suitable silver flake consists of particles which have been pressed into flake form and has the following characteristics:

A minimum of 99% by weight must pass through a 325 mesh screen; Volatile matter on ignitionl.6% by weight minimum;

' Total silver-98.0% by weight minimum;

Water covering capacity2500 to 3500 sq. ems/gram.

Other conducting fillers include copper flake, nickel flake, iron carbonyl, ferrites and other conducting and semiconducting compounds.

The epoxy resins employed in the present invention are usually prepared by condensing epichlorohydrin with hisphenol and such condensates are preferred for use in the invention. However, other 1,2-epoxy reactants, such as l-methyl chlorohydrin, l-ethyl chlorohydrin, l-butyl chlorohydrin, l-isopropyl chlorohydrin, Z-methyl chlorohydrin, 3-methyl chlorohydrin may be used in place of the epichlorohydrin. In general, one may use any 1, Z-epoxide of the general formula wherein R is preferably hydrogen or lower alkyl but may be any other hydrocarbon radical. Other dihydric phenols, such as bis-(4-hydroxyphenyl)-1,l-ethane; bis-(4- hydroxyphenyl)-l,1-isobutane; bis (4 hydroxyphenyl)- 2,2-butane; bis- (4-hydroxy-2-methylphenyl -2,2-propane 1,S-dihydroXynaphthalene, etc. m'ay'be substituted for the bi'sphenol. Glycols (ethylene glycol, diethylene glycol, etc.), glycerol and other polyalcohols may also be used in place of this reactant.

Where solvents for the epoxy resins are utilized, they may be any of the inert solvents for these resins. Thus, the solvents include those of the ether-alcohol type, such as the Carbitols (i. e. diethylene glycol mono'ethyl ether) and Carbitol and its acetate; and the Cell'o'solv'e's (.i. e. ethylene glycol monoethyl ether) and their acetate as well as keto'nes (i. e. acetone, methyl ethyl ketone, diethyl ketone, methyl isobutyl ketone, cyclohex'anone, etc.). In addition, materials, such as methyl acetate, ethyl acetate, cy'clohexan'ol, ethyl lactate, diacetone alcohol, n-butyl acetate, dioxane, tetrahydrofurfuryl alcohol, chloroform and ethylene dichloride may be used.

For those inks used in processes involving a screening operation it is desirable to employ solvents having a boiling point above about 175 C. at atmospheric pressure. However, where the inks are applied by painting or spraying, l'o-wer boiling point solvents may be utilized.

The resins used in the preparation of the inks of the present invention are preferably cured (cross-linked) and the agents used to effect the curing are desirably ureaformaldehyde, phenol formaldehyde and melamine-formaldehyde resin in admixture with the epoxy resin for subsequent cu'ring. Also useful as cut ing agents are the silicones and dicy-andiamide.

Other curing and cross-linking agents include polyamines having at least two primary or secondary amino Those polyamines which have primary amino groups. 7 groups are particularly desirable and of these agents, hex'amethylene 'diamine is especially effective. 'Mon-o primary or secondary amines may also be used for curing.

For example, vpiperidine can be utilized in cases where short pot life at room temperature can be tolerated. In most cases, however, it is desirable that the earing and cross-linkingfo'f the resins be effected by the use of compounds which "do not substantially react at temperatures less than about 75. If possible, solids are employed which do not dissolve in the resin mix until a temperature of at least 100 C. is reached. Such solids are present as particles suspended in the ink and are slowly released for reaction. Preferred among such agents are dicyandi'amide and'melami'ne.

Additional curing agents include dibasic acids and their anhydrides, such as butanedioic, octanedioic acids, phthalic anhydride, maleic anhydride and succinic anhydride as well as drying oil (e. g. linseed oil) fatty acids. Polyhydric alcohols, such as butanediol and decam'ethylene glycol may also be used. Furthermore, organic isocyanates, such a hexamethylene diisocyanate, toluene- 2-4-diisocyanate and methyl-ene-bis-(41phenyl) isocyanate may be used as curing agents.

The term epoxy resin, as used herein, of course, includes modified epoxy resins. Thus, resins treated with monobas'i'c acids at elevated temperature to 'esterify the alcohol or alcohol yielding groups in the polymers are included. Also, where desirable the epoxy resins may be combined with other resinous materials "to obtain the desired property blends (i. e., alkyds, polystyrenes, etc. may be incorporated in "the binding material).

The examples which follow specifically show the preparation of the resistor "inks of the invention as well as the structures obtained using them. It is to be understood, however, that these examples are merely il ustrative of the invention and are by no means limitative thereof. Where parts are designated in the examples or in other places in the application, they are always parts by weight.

130 C., a specific gravity of l.l'46, refractive index of 1.60, an epoxide equivalent of 1600 to "1900 and an equivalent weight of 190, and prepared by condensing epichlorohydrin with bis-phenol) is added to 35 parts butyl Carbitol acetate, after the resin has been ballmilled to a powder sufficiently fine to pass through a 20 mesh screen. The mixture is heated (e. g. to a temperature about 85-90 C.) in order to facilitate solution, then cooled to room temperature. Then, 30 parts butylated urea-formaldehyde resin (50% solids) is added to the epoxy resin solution and 36 parts silver flake and 9 parts calcined lamp black are added to the dissolved resin mixture. The resulting mixture is then stirred by hand and/or mechanical stirring until the dry materials are thoroughly wet (small quantities of a non-ionic we-tting agent, e. g. Sharples non-ionic 2543, may be added to facilitate Wetting) and an adequate amount of acetone (or any other low boiling solvent, such as methyl ethyl ketone) is added, if necessary, to attain the desired viscosity. The wet mix is then ball milled at room temperature until uniformity is attained (1 to 10 days) and the resulting ink is then removed from the ball mill and the low boiling solvent, it added, is separated by vacuum distillation. At this point additional amounts of butyl Oarbitol acetate (or any other high boiling solvent) may be added to obtain the desired consistency and the ink obtained provides a very low masterbatch having a resistance of approximately 2 ohms.

(B) Low master-batch Twenty parts epoxy resin (having a melting point of 130 6., a specific gravity of 1.146, refractive index of 1.60, an epoxide equivalent of 1600 to 1900 and an equivalent weight of 190, and prepared by condensing epichl'orohydrin with bis-phenol) is added to 35 parts butyl 'Carbitol acetate, after the resin has been ball milled to a powder sufficiently fine to pass through a 20 mesh screen. The mixture is heated (e. g. to a temperature about 85-90 C.) in order to facilitate solution, then cool-ed to room temperature. Then, '40 parts butylated urea-formaldehyde resin (50% solids) is added to the epoxy resin solution and 20 parts lamp black and 20 parts talc are added to the dissolved resin mixture. The resulting mixture is then stirred by hand and/ or mechanical stirring until the dry materials are thoroughly wet (small quantities of a non-ionic wetting agent, e. g. Sharples non-ionic 2543, may be added to facilitate wetting) and an adequate amount of actone (or any other low boiling solvent, such as methyl ethyl ketone) is added, if n'e'ce'ssary, to attain the desired viscosity. The wet mix is then ball milled at room temperature until uniformity is attained (1 to 10 days) and the resulting ink is then removed from the ball mill and the low boiling solvent, if added, is separated by vacuum distillation. At this point, additional amounts of butyl C-arbitol acetate (or any other high boiling solvent) may be added to obtain the desired consistency and the ink obtained provides a very low masterbatch having a resistance of approximately 600 ohms.

In order to obtain the desired resistance value, adequate amounts of the very low master-batch ink are mixed with complementary amounts of the low masterbatch ink. The mixing may be carried out by 'hand but is preferably accomplished by mechanical mixing in order to attain the desired uniformity. Following are the values obtained when such mixtures were prepared:

Mastcr- Masterbatch #1, batch #2, Average,

ohms ohms ohms 100 parts Very Lo'w/O parts Low 1. 5 1.8 117 70 parts Very Low/30 'parts Low--. 14 17 15, 5 parts Very Low/40 parts Low- 65 65 50 parts Very Low/50 parts Low 97.5 45 parts Very Low/55 parts Low; 200 192.5

The resistors prepared using the special inks of .the present invention may be manufactured by any of the Although non-porous bases are normally used for de-' position of the resistance material, a particularly useful form of resistor can be obtained by coating a semi-porous and flexible inorganic base with the resistor ink followed by attachment of the coated base to the fixed base in any desired position or configuration. For this purpose, base materials such as fine glass fibre, asbestos, fibre paper, glass cloth, etc. may be used. The glass cloth is highly effective especially when used in thicknesses of the order of 2 to 5 mils. The ink is normally applied to one side of the flexible cloth, then allowed to dry in order to remove the low boiling solvent, leaving a tacky resinous surface. This coated cloth may then be applied immediately to a base or stored for subsequent use in rolls protected by polyethylene or polytetrahaloethylene film material. After application of the tacky surface of the coated flexible base to the fixed base (and removal of the last traces of low boiling solvent), the resin is cured by heating. A protective resin coat may then be applied to the exposed and uncoated surface of the flexible base. The resin used for this purpose is preferably, but not necessarily, the same as the one employed in the resistance ink. Other flexible bases which may be used include those prepared from regenerated cellulose, poly-'- amides (e. g. nylon type), polyesters (e. g. Dacron type) and polyacrylonitriles. These may be rolled up or lami-- nated to other layers after processing of the resistance layer and attaching terminals. Normally, the printed resistors (or circuits employing such resistors) are provided with a cover coat. This is a protective resinous insulating housing, the resin usually being of the same type as that used in the ink. The cover coat resin will normally contain a non-conductive filling material and whatever other additive materials are required to obtain the desired adhesive, thermal expansion, etc. characteristics.

Another method of preparing resistors according to the present invention comprises laying down of the resistor on the insulating base, using an ink containing a resin having no cross-linking agent. The screened or printed resistor so formed may then be dipped in a solution of the desired crosslinking agent of, if possible, vapors of the cross-linking agent (such as ammonia or certain amines) are applied. Such procedure makes possible the use of resistor inks having infinite shelf life and ones which may be modified, as desired, whenever the manufacture of resistors is contemplated.

As another embodiment of the invention, a protective wax or resin may be incorporated into the resistor ink which will blend out of the resin during curing and thereby protect the resistor surface from adverse effects of moisture, humidity, etc. This is accomplished as a result of the reduced solubility of the cured resin.

In order to determine the effectiveness of the resistor inks of the invention and the resistors prepared therefrom, several tests were conducted. The results of these tests are tabulated below. The resistors for use in making these tests were prepared by blending the respective proportions of the low and very low? masterbatches for 12 minutes (2 minutes by hand and minutes by power), screening onto a barium titanate ceramic base having silvered terminal areas and curing for one hour at 250i3 C. A critical feature in the printed resistor art is the process change which the resistor undergoes after it is adjusted to a certain desired value. These resistors were abraded with a glass fiber eraser to within 8 to 10% of the assigned value for units of 10 and 25 ohms and to Within for higher resistances. Thereafter the terminal areas were fluxed and leads soldered thereto, degreased with an ethyl alcohol dip, cover coated with a phenolic resin and wax impregnated.

Tolerance limits.In order to determine the tolerances obtainable in the invention, resistance values were as-f signed and 60 resistors were produced for each of the assigned values. The following chart is a summary showing the assigned resistor values and percentage of resistors falling within each of three tolerance limits. The percentages are representative figures obtained for a total of 60 resistors for each value and clearly show the remarkable uniformity of process change.

RESISTOR VALUES (ASSIGNED) [Ohms] 1 Parts bv weight. L-low masterbateb. VLvery 10 .v" masterbatch.

The resistors exhibit slight positive aging after a five week test period. Thus, changes of about 1.0% result in resistors of 1075 ohms and changes approaching 2.5% are found in 300 ohm resistors. The average change is slightly less than about 2%.

Temperature c0e1ficient.Temperature 'coefiicient data obtained from these resistors show that values. of 80- ohms or less yield a positive temperature coefficient. Resistor values greater than 80 ohms have a negative temperature coeflicient over the temperature range of 25 C. to -55 C. and a positive temperature coefficient over the 25 C. to 105 C. range. All resistors showed excellent retrace characteristics. The figures listed on the following page are the average values of ten resistors for each given resistance value.

tion Serial No. 288, 305, filed May 16, 1952, now U. S. Letters Patent 2,795,680.

This invention may be variously otherwise embodied within the scope of the appended claims.

What is claimed is:

.1. A printed resistor which comprises an insulating base and an adherent layer consisting of a cross-linked epoxy resin binder obtained by condensing a bisphenol with an epoxide of the general formula wherein R is selected from the group consisting of hydrogen and a lower alkyl radical, and a mixture of finely divided silver and carbon conducting particles deposited on said base, said finely divided silver being present in a proportion from about 5% to about by weight of said mixture, and terminals attached to said layer.

2. The resistor of claim 1 wherein the binder is an epoxy resin cross-linked with butylated urea formaldehyde resin.

wherein R 'is selected from the group consisting of hydrogen and a lower alkyl radical, in a high-boiling solvent for said resin, a mixture .of finely divided silver and carbon conducting particles suspended in said solution, said finely divided silver being present in .a proportion from about to about 85% by weight of said mixture.

-4. The resistor ink of claim 3 wherein the epoxy resin is cross-linked with butylated urea formaldehyde.

5. A printed resistor which comprises an insulating base and an adherent layer consisting of from about 95% to about 50% by weight of a binder obtained by condensing a bisphenol with an epoxide of the general formula wherein R is selected .from the group consistingof hydrogen and a loweralkyl radical, and from about 5% to 50% "by weight of .a conducting .mixture consisting essentially of .finely :divided silver and carbon conducting particles, said adherent layer deposited on said base, and terminal points attached to said adherent layer.

'6. A resistor ink consisting of the condensation prodnot of a bisphenol and epichlorohydrin with a hardener and an electrically conducting mixture of finely divided particles of silver and carbon, said finely divided silver being present in a proportion from about 5% to by weight of said mixture, in a solvent for said resin having a boiling point above C.

"7. A resistor ink having an epoxy resin obtained by condensing a bisphenol with epichlorohydrin combined with a hardener and a mixture of finely divided electrically conducting particles including lampblack and from about 5% to 85% silver particles by weight of said mixture, the finely divided particles being from 5% to 50% of said ink, and said mixture being dispersed in a solvent for Said resin.

References Cited in the file of this patent UNITED STATES PATENTS 2,528,360 Greenlee Oct. 311, 1950 2,683,673 JSilversher July 13, 1954 2,691,007 Cass Oct. 5, 1954 2,692,321 Hicks Oct. 19, 1954 OTHER REFERENCES :Printed Circuit Techniques,--Natl .Bureau of Standards Circular 468, Nov. 15 1947, pp. 5-79.

Paint Oil and Chemical Review No. 9, 1950, v. 1113, issue .23., pp. .1,51;8, 48 and 59.. 

1. A PRINTED RESISTOR WHICH COMPRISES AN INSULATING BASE AND AN ADHERENT LAYER CONSISTING OF A CROSS-LINKED EPOXY RESIN BINDER OBTAINED BY CONDENSING A BISPHENOL WITH AN EPOXIDE OF THE GENERAL FORMULA 